Frequency measuring apparatus



sept 1 2, 1944- H. v. HERMANSEN 2,358,127

FREQUENCY EASURING APPARATUS Filed nee. 1v, 1941 INVENTOR' Hg/vRY v. HERMAA/.sE/v y ATTORNEY.

Patented Sept. 12, 1944 FREQENCY MEASURING .LiPPARArlUsy Henry- V. Hermansen, Baltimore, Md., assignor to Bendix Aviation'Corporation, South Bendflnd.

a corporation of Delaware Application December 17, 1941,'Serial No. 423,363

. o Y y (Cl. Z50-39) 7 Claims.

'I'his invention relates to oscillatory systems and more particularly to oscillator circuits incorporating a visual beat frequency indicator for comparing theY frequency of the oscillations with the frequency of externally generated alternating voltages.

One of the most precise methodsof determining an unknown output frequency is to combine that frequency with a known frequency from a calibrated source of energy to produce beats.

The known frequency may thenbevaried until a Zero beat is obtained, at which time the output frequencyfrom the tw'o sources is equal.` Conversely, the unknown frequency may be varied until zero beatis obtained, thus adjusting the unknown source to provide a frequency equal to that .derived from the standard calibrated source.

The detection of the zero beat is often performed by an electro-acoustic transducer, .such as a telephone headset, connected to a point in the circuit where the two frequencies have been combined. A serious deficiency in the accuracy of Vsettings -performed with the aid of such transducers is occasioned by the rapid drop in trans# ducing efciency at very 10W frequencies. Beats of the order of two or three per second are be-A yond detection.

A satisfactory solution hasbeen developed for those applicationsl where a great amount of pow.- er is available ateach of the two frequencies, for

it is only necessary to addthevoltages at. ther two frequencies and pass the resultant into a rectifier whose direct currentv output electrodes are connectedto a direct current meter of the.

DArsonval type. The deflection Vof the. meter swings slowly back and forth at thedifference frequency between the two sources of energy.V

This answer, while satisfactory for. installations where large amounts of power are available, stilly leaves unsolvedthe problem of supplying equally..

accurate indicati-ons,V where smaller amounts of power are available, without bulkyrand expensive amplifiers.

One of the principal objects of thisinventon is to provide improved apparatus for visually de.-V tecting low beat frequencies. Y

'Another object of= this invention is to provide apparatus combining the active elements of an oscillator and a beat frequency indicator to re'- duce the bulk and weight of frequency measuring apparatus. .t

Still. another object of this invention is to provide improved. apparatus .for .detecting low beat frequencies between a low power source Yof energy and a standard source of frequency.

` trol electrode 6 isa small rodlocatedbetween the l cathode and the target.A When the'rayjcontrolj electrode is made negative, electrons are excluded .55 from the arc sector under its controLwhil'e, when.

The above objectsyand advantages ofthis in,- vention. areaccomplished by utilizing an indi'- cator of the variable shadow area type torespondto plate current variations in any of the well known types of oscillator circuits. This has enabled the use ofV a single commercially available tube to perform both thefunctionof oscillationgenerator andbeat frequency indicator.

' Other objects and advantages will in part. be'

disclosed and in. part be obvious when the following specification is read Vinconjunction with the drawing in which:

Figure 1 is a schematic diagram'of form ofthe invention. A c

Figure 2 is a schematic diagram of an alternative form of the invention. c

Figure 3 isa schematic diagram of' another alternative form Iof the invention.

a preferred Figure 4 is a schematic diagram of 'still' an,-

other alternative form of the invention.

It is to Vbe, understoodl .that thisdrawing is intended to illustrate some ofthe many formsin which Ythe invention. may be utilized and is not to comprise alimitation. in the content or scope ofthe invention.

In the drawing, 'like partsare designatedv by like reference characters* y l Referring to Figure 1,k the'vacuum tube l includes the cathode 2 with the thermally associated heater 3. One,` portion of the cathode 2' provides emissionr for the set of elements comprising the control grid: 4 and the anode 5,.the combination y constituting .the familiar trlode.v

Another portion of the cathodey 2. provides emission for the set of elements including the ray control electrode 6 and thephosphor coated target' l, the combination comprising a well known type of visual electronic indicator. .The vacuum tube known commercially astype 6E5 contains allof the above elements and has' been successfully employed in the practice of the invention. Inpresent day vacuum tubes of this type, the target 1' surrounds. the cathode 2. and is an annularly shaped piece of metal which is dished in form,

so that electrons leaving the` cathode normally:

will strike the inner surface which may be. readf ily viewed from the larger endjo-f the dish. The inner surface of thetarget is coated with a material which glows under the impact of electrons, commonly known as aphosphor. yThe ray conmade positive, the ray controlelectrode permits passage of electrons to all parts of the target. Thus, as the ray control electrode is made more negative, a larger and larger sector of the glowing phosphor is thrown into shadow, since the electrons are excluded from a larger and larger sector through the action of the ray control electrode.

energy from the source I5 to the oscillating circuit is shown. The vacuum tube I having the Returning V'again to :Figure 1,V the piezo-'elecf tric resonator is connected between the control grid 4 and the cathode 2 in parallel with the grid leak resistor 9, which may be of one megohm. The cathode 2 is also connected to the negative terminal of source I0 which `may have a potential of 200 volts, fand-the phosphorv coated target l is connected to the positive terminal of the source IIJ. vacuum tube I is connectedwithin the tube to the ray control electrode B and is also connected to the positive terminal of source II) through the parallel resonant ,circuit comprising inductance I Ifandvariable capacitance I2 in'series with the voltage dropping resistorv I3, which may have ar f resistance of l500,000 ohms. The bypass capacitor.I4 provides a low impedance path forrthe radio frequency :component of theanode current ,aroundftheresistor I3, but presents a rel-V atively high impedance :to audio and sub-audio frequencies.'v Asuitable'value for ,this capacitor is'.01 mfd. The source, rl5 having a frequency which is tcJbey determined orxadjusted may be connected to the grid circuit of the triode through the capacitor I6 of 5.mmfd. Since heater circuits of manyvarieties are well known to those skilled in the art, the source of heater energy and the connections theretol have been omitted fromthis and the remaining figures.

Neglecting the source yI5 for the moment, the triode section of the vacuum tube I in this circuit operates 'as. theusual .crystal oscillator inV whichthe parallel-'resonant circuit is tuned to a The anode 5 of the same elements as previously enumerated, is employed. The piezo-electric resonator 8 and the grid leak resistor 9 are connected in parallel, and this combination is connected between the control .grid 4 and the cathode 2 in series with the coupling coil'I'I. Inductively coupled to coil Il is a coil I8 which is connected to the source I5,

whose frequency is to be tested or adjusted. The

parallel resonant circuit comprising inductance .-II and variable capacitance I2 is connected befrequency'somewhat higher than the natural res onance frequency of the resonator 8 to produce-f' InE an oscillator` of thejselfbiased grid leak type, it is well known .that a,

self-oscillation.

dropin anode current accompanies a decrease in lthe load on the oscillating .l circuit, while' an difference jbetween the two frequencies,Y whichA will'be referred to as fthe beat frequency, and the anode current variations produce a periodic Y which may be provided with a cooperating pointer'4 voltage'across the. resistor.` I3, at the beat fre-V quency. Because ofthe direct connection between the'anode5 and the ray control electrode 6 ,the beat frequency also appears on the ray,Y

control Yelectrode and results in a corresponding periodic fluctuation 'in the widthdof the shadow sector V.on .the target 1. Source I5 may now be set to isochronism with the crystal oscillating frequency by adjusting it so that the shadow angle viuctuat'ions cease, at `which time the.fre

quenciesare identical. The great advantage of this'j method is 'that` 'there is no lower ,frequency limit of observation,` Y In Figure 2, a' different method of transferring and subtracted from the oscillating circuit caus-l ingY periodic fluctuationsin the oscillation am-V An oscillator of the grid leak type is4 plitude.

tween the positive terminal of the source I0 and the target l, while the voltage dropping resistor I3 Vand bypass condenser I4 are connected in parallel from target 'I to the anode 5.

The operation of the crystal oscillator takes place in the manner already described. With the 'source I5 connected as shown in Figure2 and supplying a frequency differing from the crystal frequency, the oscillating voltage on the Y control gridis alternately reinforced and op'- posed, thus giving rise to anode current varia-4 tions atV the beat frequency between the two frequencies. As before, the ray control Vpoten-1 tial is correspondingly varied, resulting in a fluc-v tuating shadow pattern. Observation of this pattern enables the determination of or adjust ,-ment to isochronism between source 30' ,the'crystal oscillating frequency. ,The presence.

of oscillatory voltage on the target 'I has been found to leave the operation of the indicator' substantially unaffected.

,l Turning .no wto Figurei, the invention is` shown applied to an oscillator circuit'frequently referred to as the Pierce circuit, with some modi cations in the grid circuit. The vacuum tube IV having the same elements previously enumerated is employed. The target 1 is connectedto, the positive terminal Vof the source I0, and the' anode 5r is connectedV to the target 'l through the resistor I3. The gridleakresistor 9 is connectedY between control grid 4 and the cathode 2, while the piezo-electric resonator 8 is connected be tween the control grid 4 and the anode 5 in par- 'v allel with the' variable capacitor I9. The series combination of inductance 20 and capacitance 2I is alsoconnected between the control grid .4 and. the cathode 2, which is connectedto the negative terminal of thesource I I'I. A suitable value'forA inductance 20 is 1 millihenry, and for capaci.

tance 2I is 1 mfd. 'Source I5 is coupled'to the control grid 4 through the capacitor I6 as previ.,

viously described.

The' operation of the triode section tor v8 occurring by virtue of the capacitive nature ofthe anode load. The frequency of oscillation'.

may be varied by adjustment of capacitor I9,

and scale calibrated in terms of the oscillating frequency for any position d of the capacitor. When source I5 has a frequency differing from the oscillating frequencyofthe'triodeisection of vacuum tube I, energy is alternately added to possessed of a self-stabilizing characteristic as a result'4 of two eiectswhich oppose eachother.

An increase in oscillation amplitude taken by itself would cause anincrease in anode current, but an Vincrease in oscillation amplitude gives rise to additional grid current and consequent addi-V as an os-Y cillator takes place in thewell knownvmanner, sustained oscillation of the piezo-electric resona tional -'-bias Y' with the result :that Y the `oscillation amplitude returnsfto nearly-the lformer value. land the anode-current :suffers fa decrease in amplitude. 'In'ductance 2011's- Eselected vto present high impedance'to 'radio llfrequency currents .iandllow impedance? to audio frequency currents, and .the capacitor .-'2I ,i-,whichlis connected inseries withjt', is fchosenfto provide-ariairly large .time constant inconjunction with thegridleakresistor :9. iThe capacitor12l ldiminishes'the:grid bias variations andthefaudio or 'sub-audio. frequencyf'voltagefavpi-r pearin'g across-Itho resistorfl3 istherebyfgreatly increased with an attendantincrease in the `sensitivity f of indication, -as the vshort-term, "seliasta-v bilizingnctionof theoscillatorhas now been-:eliminated, so that aneincrease-fin oscillation amplitudeibrings about a f considerable increase Tin anodecurrenteand vice versa. Asfbefore,v the beat frequency voltage appearing onthe ray. control electrode makes the'shadowzwidth on Ythe target 1- vary periodically. By adjustment f of :the cal Y pacitor I 9,*the :oscillation frequencyof the trio'de;

section `may nowfbe adjusted to zero v-beatI with the: source: 'I'5f by I(itrservation of 'the shadowv patterxrf-and when" isochronism `has Abeen Vattained', the rfrequency of the vvsource fI 5 may-be immediatelyfobtainedby reference to therequired set; ting of thelcapacitor I 9.

The schematic diagram f of Figure 4 `s hows the invention-as practiced using two separate vacuum tubes, the differencef'between this and the preceding diagramV residing vin the elimination @of radio ffrequency potentials from .the 1ra-ycontrol electrode. Inthisembodiment, the vacuum tube 22 fis utilized! iasnanioscillator :with the piezo -^electric :resonatora .connected -betweenf` the. anode f2.5

and the control,` gridv 25thereof. The iresistorgiS,

connectedbetweenV the :control ,grid .25 .and .the cathode 22 provides a direct icurrent; grid return and thecondenser .2l connected across resistor 9 in series with the radio frequency choke acts to delay the rate of change of voltage across resistor 9 as outlined in the discussion of Figure 3. The source of unknown frequency I5 is connectedto the control grid through the coupling capacitor I6 and the energy conveyed thereby serves to modify the operation of the oscillator circuit for the purposes of frequency comparison. anode 26 is connected to the other terminal of the piezo-electric resonator 8 and to the choker-21 having high impedance to oscillation frequencies and low impedance to audio and sub-audio frequencies. The other terminal of choke 21 is connected to the positive terminal of the source IU through the resistor I3. The indicator tube v28` includes the cathode 29 with associated heater 33, twin ray control electrodes 3I, 32, and the twin phosphor coated targets 33, 34 which are connected to the positive terminal of the source Ill. The ray control electrodes 3l, 32 are connected to the common terminal oi the resistor I3 and the choke 21, and radio frequency voltages are eliminated from thispoint by thebypass capacitor 35 connected between the ray controlV electrodes 3l, 32 and ground. A capacitor of .0l mid. hasl been found satisfactory in the position of capacitor 35. The cathodes 23 and 29 are b 'oth connected to the negative terminal of the source l The vacuum tube 22 may be of thetypeknown commercially as 6.15, and the commercial` type 6AD6 may be employed in the position of vacuum tube 28.

The operation substantially identical with that of Figure 3, the separation of the indicator and oscillator tubes onds. f

of the apparatus of Figure 4 is I'It isalsof'considered -that la single cathode, i dif-v ferentf portionsl of which. supply electronic emisa sion 'tofdifferet-fsetsof felements, is the fequiva#y 1ent fof :a`r mum-berief -fseparately#constituted 'amil'r heatedfcathodes@which-.aresoperably connected; i "It willbe obvious-'that many changes: and modi?- cations -maylbe'made' in" tha-invention without departing from the spirit'thereof asfeipress'ed-fin. `the 'foregoing'fdescription 'andfin the" appended claims: Y 'WhatiIcl m -lx'lh'e @combination 1- of ian oscillatory system" comprising':aisourceo'ffsustainedv oscillations, saidT- system=includingan electric'fdischarge devicev'hav ing-'input fand-outputelectrodes, means forimv pressing. aiternatingcurrentfenergyfrom a second; sourcecn said# oscillatoryfsystem, -a second- -electricdischarge'deviceihaving acathode, a frayfco'ntrol electrode'and' a phosphor *coated targetba connection-fbetween said output 'electrode Y and said ray-control-electrode,*and an envelope common to -bother-said electric discharge devices.

251-11 lcombination,-an electricdischarge 'device element offering high impedance to audio and` sub-audio frequency variations connected in series with said inductive impedance, a connection between said cathode and the negative terminal ofsaid source, a second electric discharge device having a cathode, a ray control electrode and a phosphor coated target, a connection between said first mentioned cathode and said second mentioned cathode, a connection between said anode and said ray control electrode, a connection between the positive terminaloi said source and said target, an independent source of alternating current energy, and means for impressing energy from said independent.v source on said electromechanical resonator.

3. In combination, an electric discharge device having a cathode, a control grid and an anode, a power source having positive and negative terminals, an electromechanical resonator connected t between said-control grid and. said anode, a high impedance connected between said control grid 'and saidgnegative terminal, a highimpedance connected between said anode and said positivel terminal, a connection between said cathode'and said negative terminal, a second electric discharge device having a cathode, a ray control electrode alternating current energy, and meansvfor im-y f .Y 'K

pressing energy from said independent sourceon saidelectromechanicalresonator.,

4.- In combination with a vacuum Ytube oscila;

' cycles per second` connected to saidimpedance,

an *independent Ysource of alternatingcurrent energy, and means for impressing-,energy `from said` independent' source on the control grid of said oscillator. f Q Q V5,. The-combination of; an;electric discharge tube having a cathode, a control grid andan anode, a piezo-electric resonator, connectedto said control grid, a resistance connected between said control grid and said cathode, a series comf- .bination of inductance and I capacitance con- Y nected between said control grid and said cathode,v

said inductance having an impedance high to the natural frequency ofsaid piezo-electric resonator and low to audio frequencies, said resistanceand capacitance having a time constant greater than one twenty-fifth of a second, a visual indicator responsive to frequencies lower than tWenty-ve cycles per second connected to ,an elementA of said electric Vdischarge tube, 'an/independent sourceof alternating current energy, and means for impressing energy 1 from :said independent sourcexon the control grid ofsaid electric dischargetulzveUr:H L

-j6. 'I'he combinationY of an/electric discharge Ytube having `afcathode, acontrol grid andan anode, ay piezo-electricV- resonator:V connectedv to said control grid', a resistance connected between said control grid and said cathodeya series :com-4 bination of inductance and capacity` connected between said controlV grid and said cathode, .Said inductance having an impedance high "to, Ythe natural frequency of said piezo-electric resonator'- and lowv to audio frequencies, said-resistance and capacity having a time constant greater than one twenty-fifth of av second, an impedance -to audio and sub-audio frequencies connectedY in series with the anode circuit of said tubeya visual indicator responsive'to frequencies lower 'than twenty-five cycles per second connected to said impedance, an independent source of Valternatingv current energy, and meansforimpressing energy from said independent source on the control grid of said electricdischarge tube.` 1 Y '7. In frequency measuring apparatusfanoscih; lator including an electric discharge device-hav-V ing a cathode, a control grid and an anode, a source of direct current energy having positive and negative terminals; a resistorand anin-L ductor connected in series'betweeny said positivej terminal of said source and said anode,- said inductor being connected to said anode and said resistor being connected to said positive terminal; a Ysecond electric discharge device having a cath# ode, a ray control electrode and aphosphorl coated target, means connecting vsaidv ray'control electrode to the junction. between Vsaid resistor and said inductor, means connecting saidsecondmentioned cathode to said first mentioned cathode, means connecting' said target to said positive' source terminal, a capacitor connected between said ray control electrode i andthe cathode oiV said second mentioned electricv dischargedevice, a source of alternating current energy, and means for impressing energyfrom said alternating current source on an element of said rst mentioned y electric discharge device. i

' HENRY V. HERMANSEN'.: 

